Shopping Cart missing in checkout with multisite installation of MarketPress w/ Gridmarket

I noticed a few posts on here about the items in the shopping cart going missing after going to the checkout, but none of the entries have been of any help. I am having the same problem and do not know how to resolve it.

I'm running a multisite installation that is utilizing subdomains, but has been installed within a subfolder on the site. I thought this might be a cookies issue, so I've been playing with the COOKIEPATH, SITECOOKIEPATH definitions in the wp_config to no avail.

What should I do now?